Underwater acoustic transducers are used in communications and to aid in determining the position and navigation of submerged objects. One such method to aid in navigation is to transmit a spiral wavefront consisting of a signal having a magnitude that is nominally constant but whose phase varies linearly as a function of azimuthal angle in a defined plane. Such a spiral wavefront signal can be compared with a reference signal of constant phase to determine the bearing angle. A beacon carrying such a transducer producing a spiral wavefront may be employed to transmit signals that can be detected by multiple objects or vehicles, thereby providing a cost effective navigation aid to determine bearing angle to the beacon. The challenge is to realize and effective transducer to accomplish this goal.
It is known to those skilled in the art, that one way to create a spiral wavefront is to employ a plurality of transducers arranged around in a cylindrical pattern around a rigid cylinder backing wherein each element is driven with an incremental phase bias that is retarded with respect a neighboring element in order to produce a spiral wavefront. It is also known to those skilled in the art, that a spiral wavefront transducer may be realized by employing a transducer or array of transducer elements arranged in a cylindrical-spiral pattern with each section or segment having an incremental radial offset with respect to its neighbor in order to create a spatial phase delay in the wavefront when driven by a common signal. This approach has a discontinuity when one full revolution is reached. These approaches are discussed in U.S. Pat. No. 7,406,001 by Dzikowicz and in the publication [Ref. Hefner and Dzikowicz J. Acoust. Soc. of Am.], in which a navigation method is proposed based on an underwater acoustic beacon comprising a transducer for producing a spiral wavefront signal and a transducer for producing a reference signal of constant phase aligned along a common central axis.